1. Field of the Invention
This invention relates to in-line fluid heating apparatus and more specifically to an apparatus for warming parenteral fluids such as blood.
2. Background Art
Blood and other parenteral fluids are commonly stored at hypothermic temperatures in the range of about 2.degree. -10.degree. C. to maintain freshness and viability. Before such fluids are infused into a patient, it is common practice to raise the influent fluid temperature to nearly normal patient temperature levels of 36.degree.-38.degree. C. This is sometimes done by using water baths to warm several individual units or bags of blood simultaneously. In some cases, such as open heart surgery, it is desirable to maintain the patient at hypothermic temperatures for a period of time and infuse blood at substantially the same hypothermic temperature. As such infusions vary in rate and temperature, it is best to have in-line blood warming devices which warm the blood as delivery is made from a blood bag to the patient to conserve blood and reduce delays of off-line heating methods, such as water baths.
The rate of caloric exchange and thus the rate at which a fluid such as blood is warmed is proportional to the temperature differential between a heat emitting surface and the blood, for a given increment of time. This means that, theoretically, a fluid at zero degrees may absorb 100 calories per second when in contact with a surface maintained at 100 degrees. Similarly, a fluid at 50 degrees will absorb 50 calories per second when contacting the same 100 degree surface, and a fluid at 90 degrees will absorb only 10 calories per second when contacting that same surface. Thus, a fluid such as blood is warmed at the inlet of an in-line fluid warming device it will absorb more heat than at the outlet since it becomes warmer as it travels through the device. Accordingly, a complication in the warming of viable physiological fluids, such as blood, is a maximum safe temperature which may be used in the heating process. It is commonly known that the maximum safe temperature which may be used is in the range of 38.degree. C. This tends to impose serious constraints in some situations. For example, in emergency cases where a patient has lost large quantities of blood, the blood must be replaced quickly by rapid infusion. However, it is very difficult to warm such large quantities of blood to the desired temperatures rapidly enough using in-line blood warmers.
Since the blood which exits an in-line blood warmer must typically be 36.degree.-38.degree. C., where large quantities of blood are required very quickly as in causes of severe trauma and blood loss, the temperature of the warming apparatus is typically elevated above the 36.degree.-38.degree. C. range because heat transfer must take place more rapidly. Thus, by increasing the temperature, as the cool blood first enters the in-line warming apparatus, it will be more rapidly warmed. However, it is very difficult to maintain the proper temperatures at both the inlet and the outlet. If the temperature is too high, the outlet blood temperature may be proper but the blood may be damaged by the high temperature at the inlet as it is first heated. If the temperature is lowered, damage to the blood at the inlet may be avoided, but the desired outlet temperature may not be achieved. To date, these and other problems in the art have not been adequately solved.